The demand for electronic assemblies in the automotive, aerospace and various other industries has resulted in the annual production of millions of electronic assemblies by manufacturers in the electronics industry. Often, demand has increased to the point that additional processing equipment and floor space is required to meet the growing demand. To enhance their production efficiencies, electronics manufacturers continuously seek to implement new technologies which can increase output without a corresponding increase in capital, floor space and labor.
As is also well known in the art, electronic assemblies are often required to be capable of withstanding hostile operating environments, such as those commonly found in the automotive and aerospace industries. One practice widely accepted in the electronics industry is the use of a conformal coating which forms a protective barrier layer on the circuit board. Conformal coatings are formulated to protect the electronic assembly from moisture and dirt, as well as make the circuit devices mounted to the circuit board more resistant to vibration. Generally, conformal coatings have been composed of polymeric materials of the silicone, acrylic, urethane and epoxy families. These families can be divided into groups based on their particular systems and their curing characteristics. For example, there are two-part material systems which cure upon mixing of the two components, one-part solvent-borne systems such as acrylic and hydrocarbon resins, one-part moisture cure systems, such as urethanes, epoxies and silicones, one-part frozen premixed systems, one-part heat-cured systems, ultraviolet (UV) cured systems, and vacuum deposited materials such as PARYLENE, available through the Union Carbide Corporation. Other than the vacuum deposited materials, the above coating systems are typically applied by dipping, spraying or brushing techniques, and occasionally are deposited as multiple layers. The product design, the coating process and the process capacity will generally dictate which type of coating system can-be applied for a given application.
While the above materials have generally performed well, significant shortcomings exist with each. For example, two-part systems, such as CE1155 available from Conap of Olean, N.Y., typically have an extremely short pot life after mixing, and mixing problems inherently arise from time to time. Two-part systems typically require a solvent in order to reduce the viscosity of the system to a usable level. Such solvent-borne systems, such as acrylics, are often undesirable in that the solvent must be removed prior to handling, their volatile organic compounds (VOC) and/or ozone depleting compounds (ODC) may require incineration, and most solvents used pose a fire risk and/or require explosion proof equipment, all of which significantly increase manufacturing costs. Use of volatile organic compounds and ozone depleting compounds is of particular concern, in that volatile organic compounds are generally closely regulated by environmental agencies, while most ozone depleting compounds are being phased out by law. The use of water as the solvent overcomes the hazards noted above, but such systems typically have reduced electrical and physical properties. In addition, considerable costs are incurred to remove the water from the coating system.
Drawbacks are also associated with known one-part moisture cure systems. For example, urethane systems typically require seven days for full and stable electrical properties to be attained, epoxies cure slowly and will only develop thin cross-sections after cure, and silicone systems have about 5 to about 20 percent VOC from a condensation reaction which may be very fast, but the depth of cure is limited. In addition, one-part systems which are borne with solvents such as xylene and toluene, for example an acrylic system available from Start Manufacturing under the name PC101, have stringent shipping restrictions and pose health and safety concerns. Shortcomings of one-part frozen premixed systems include a short pot life at room temperature, necessitating that the system be stored frozen, though often such systems also have relatively short frozen mixed shelf lives. One-part heat cured systems have the disadvantage of requiring an extended period at an elevated temperature to cure, increasing manufacturing costs based on time, energy and inventories. UV-cured materials are generally expensive and, due to shadow cure deficiencies, typically require a secondary cure mechanism for areas on the assembly which are not be exposed to sufficient UV energy to cure the coating. Often, such areas are liquid after the UV cure, and the secondary cure, such as a moisture or heat cure, occurs slowly. Finally, vacuum deposited coating materials such as PARYLENE are extremely costly and are also costly to process.
Generally, each of the above coating systems also can be characterized as having processing disadvantages based on inventory and long processing times due to required mixing conditions and limitations, the limited methods by which these systems can be applied, and the extended period and/or equipment required to cure the coating systems. In particular, production rates of electronic assemblies are typically limited by the cure schedules of the conformal coating systems used. Consequently, manufacturing demands such as capital, floor space, labor and process inventory can be artificially high for the production of electronic assemblies which require environmental protection with a conformal coating.
In addition to the processing drawbacks noted above, conventional conformal coating systems also are capable of creating reliability problems with the electronic assembly. Specifically, the flow or placement of the coating material on the circuit board often cannot be readily controlled, In particular, the conformal coating materials often flow beneath the circuit components mounted to the circuit board, and fill at least a portion of the gap between the component and the circuit board to the extent that the gap is completely bridged. When this occurs, differences in coefficients of thermal expansion create stresses in the leads and solder joints which electrically connect and physical attach the component to the board. As a result, the expected life of the solder joints may be significantly decreased, at times on the order of up to a 75 percent loss in expected life. Factors which effect the solder joint stress include the coating thickness, its bulk modulus of elasticity, the physical characteristics of the component and its leads, the size of the gap between the component and the board, and the coefficient of thermal expansion of the component, its leads, the board, and the solder. Notably, current integrated circuit packaging trends are toward integrated circuit packages with lower standoffs and less compliant leads, therefore resulting in packages which further complicate the coating process.
From the above, it can-be seen that it would be desirable if a coating material were available which could overcome the above-noted shortcomings of conventional conformal coating systems. More specifically, such a coating material would not require the use of a solvent so as to avoid the environmental and safety hazards associated with solvents, as well as the additional costs incurred to properly address such hazards. In addition, it would be desirable if such a coating material could be applied using techniques which enable more selective application in order to avoid bridging the gap between the circuit components and the circuit board of the electronic assembly.
Accordingly, what is needed is a conformal coating system which does not require the use of solvents, can be applied in a manner which avoids bridging the gap between the circuit components and the circuit board, and avoids cure-related problems while reducing processing time and process inventory. It would also be desirable if the cost of such a coating system were relatively low in order to further minimize manufacturing costs.